Adhesive composition comprising (meth)acrylate polymer and epoxy resin

ABSTRACT

An adhesive tape comprising an energy beam transmittable base sheet having a surface tension of not more than 40 dyne/cm and an adhesive layer formed on one surface of the base sheet, the adhesive layer comprising a (meth)acrylate polymer, an epoxy resin, a photopolymerizable low molecular weight compound, a heat activatable latent curing agent for the epoxy resin and a photopolymerization initiator for the photopolymerizable low molecular weight compound. The adhesive in the adhesive layer is curable with an energy beam and the so cured adhesive develops tackiness again when heated. When the tape is used in processing a semiconductor wafer, it serves as a dicing tape for holding the wafer in position during the dicing step. Each piece of the diced and cured adhesive layer, that is attached to each chip and capable of being tackified by heating, provides an adhesive required for securely mounting the chip on the lead frame in the die-bonding step.

This application is a division of Ser. No. 07/653,232 filed Feb. 8,1991, now U.S. Pat. No. 5,110,388, which is a division of Ser. No.07/380,548 filed Jul. 14, 1989, now U.S. Pat. No. 5,118,567.

FIELD OF THE INVENTION

The present invention relates to a novel adhesive tape and use thereof.More particularly, it relates to an adhesive tape suitable for use indicing a semiconductor wafer into chips and die-bonding the chips on alead frame, and to use of such an adhesive tape.

BACKGROUND OF THE INVENTION

Wafers of semiconductor material such as silicon and gallium-arsenichaving formed thereon integrated circuits have a relatively largediameter. In the production of integrated circuits (IC), such a wafer isadhered to an adhesive tape sometimes called a dicing tape, diced intochips (IC chips), which are then released (picked up) from the dicingtape, and adhered to (mounted on) a lead frame by means of an adhesivesuch as an epoxy resin.

For dicing tapes suitable for use in such a wafer processing technique,is required that they should exhibit a strong adhesion to thesemiconductor wafer in the step of dicing the wafer into IC chips on theone hand, while they should also have such a reduced adhesion to the ICchips in the step of picking up the IC chips so that the IC chips may bereadily released without carrying adhesive residues from the dicingtapes. Thus, in prior art dicing tapes adhesive and releasingproperties, which are conflicting, are required, and use of a dicingtape whose adhesive and releasing properties are not well balanced inwafer processing, invites such a problem that the dicing and/or pickingup steps of the process cannot be smoothly carried out. In particular,when a prior art dicing tape is used in wafer processing, a part of theadhesive of the dicing tape is transferred to the picked up IC chips andadversely affects the characteristics of the resulting IC. Accordingly,it has been necessary to remove the undesirable residual adhesive fromthe IC chips before they are mounted on a lead frame. While completeremoval of the residual adhesive is difficult, the step of removing theadhesive not only makes the processing complicated, but also invites aproblem of environmental pollution if an organic solvent is used, forthe removal of the residual adhesive.

Furthermore, a separate adhesive such as an epoxy resin has been usedfor mounting the IC chips on a lead frame, as described in JapanesePatent Laid-open Publication No. 60-198,757. The use of a separateadhesive involves another problem such that unceasing application ofappropriate amounts of the adhesive is technically very difficult, whichresults in a case of a very small. IC chip in the applied adhesivefrequently bulging out of the chip, whereas in a case of a relativelylarge IC chip, the amount of the applied adhesive tends to be too smallto provide a desired adhesion.

OBJECT OF THE INVENTION

The invention intends to solve the above-discussed problems associatedwith the prior art and an object of the invention is to provide anadhesive tape suitable for use in processing semiconductor wafers whichhas both actinic radiation curability and heat curability, which can beused as a dicing tape in the dicing step and which can provide anadhesive in the mounting step.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention there is provided anadhesive tape comprising an energy beam transmittable base sheet havinga surface tension of not more than 40 dyne/cm and an adhesive layerformed on one surface of said base sheet, said adhesive layer comprisinga (meth)acrylate polymer, an epoxy resin, a photopolymerizable lowmolecular weight compound, a heat activatable latent curing agent forsaid epoxy resin and a photopolymerization initiator for saidphotopolymerizable low molecular weight compound.

In accordance with another aspect of the invention there is provided amethod for using the abovementioned adhesive tape, which methodcomprises the steps of adhering a semiconductor wafer to said adhesivetape, dicing said wafer into chips together with the adhesive layer ofsaid adhesive tape, irradiating the adhesive layer of said tape with anenergy beam, picking up the chips together with pieces of the dicedadhesive layer adhered thereto from said base sheet, placing the chipson a lead frame so that the respective pieces of the adhesive layer maycome in contact with said lead frame, and causing the pieces of thediced adhesive layer to again develop tackiness by heating therebysecurely mounting the chips on said lead frame.

In the method according to the invention, the adhesive tape according tothe invention serves as a dicing tape for holding the wafer in positionduring the dicing step. Each piece of the diced and cured adhesivelayer, that is attached to each chip and capable of being tackified byheating, provides an adhesive for securely mounting the chip on the leadframe in the die-bonding step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an adhesive tape accordingto the invention;

FIGS. 2 to 7 are illustrations showing states of the adhesive tape ofFIG. I in various steps of a wafer processing technique which it isused; and

FIG. 8 is a cross-sectional view of a modification of the adhesive tapeof FIG. 1

DETAILED DESCRIPTION OF THE INVENTION

The adhesive tape and use thereof according to the invention will now bedescribed in detail with reference to the accompanying drawings.

As schematically shown in FIG. 1, the adhesive tape 1 according to theinvention comprises a base sheet 2 and an adhesive layer 3 formed on onesurface of the base sheet 2. Before use of the adhesive tape it ispreferable to tentatively apply a strippable release sheet (not shown)to the adhesive layer 3 for protection thereof.

It is desirable that the adhesion strength between the base sheet 2 andadhesive layer 3 is initially high and can be reduced by irradiation toa level sufficiently lower than that between the adhesive layer and asemiconductor wafer. For this purpose pose the base sheet 2 should havea surface tension not more than 40 dyne/cm, preferably not more than 38dyne/cm. Further, suitable as the base sheet are materials which are lowin electrical conductivity and excellent in water resistance as well asin heat resistance. From these viewpoints, synthetic resin films areparticularly preferred. As will be stated later, the adhesive tapeaccording to the invention, when used, is irradiated with an energy beamsuch as an electron beam or ultraviolet ray. When it is to be irradiatedwith an ultraviolet ray, it must be light transmittable, but it is notrequired to be light transmittable, when it is to be irradiated with anelectron beam. Practically, suitable as the base sheet 2 are films ofsuch synthetic resins as polyester, polyethylene, polypropylene,polybutene, polybutadiene, vinyl chloride ionomer, ethylenemethacrylicacid copolymer, vinyl chloride-urethane copolymer as well ascross-linked films of such resins. The films may or may not be treatedwith a silicone.

The base sheet 2 may be either of a single layer or laminated. Thethickness of the base sheet is normally from 25 to 200 μm.

The adhesive layer 3 of the adhesive tape according to the inventioncomprises a (meth)acrylate polymer, an epoxy resin, a photopolymerizablelow molecular weight compound, a heat activatable latent curing agentfor the epoxy resin and a photopolymerization initiator for thephotopolymerizable low molecular weight compound.

By the term "(meth)acrylate polymer" used herein is meant polymersprimarily (at least 50 mol %) comprising structural units derived fromat least one (meth)acrylate i.e. acrylate or methacrylate. Examples ofthe suitable (meth)acrylate include, for example, glycidyl acrylate andmethacrylate as well as alkyl and hydroxyalkyl (meth)acrylates, in whichthe alkyl moiety has from 1 to 14 carbon atoms, such as methyl, ethyland butyl acrylates and methacrylates, and 2-hydroxyethyl acrylate andmethacrylate.

The (meth)acrylate polymer which can be used herein may be a homopolymerof a (meth)acrylate, or it may be a copolymer of at least two(meth)acrylates. Alternatively, it may be a copolymer of at least one(meth)acrylate and at least one comonomer copolymerizable therewithcontaining at least 50 mol % of units derived from said at least one(meth)acrylate. Examples of the comonomer include, for example, acrylicand methacrylic acid, acrylonitrile, methacrylonitrile, vinyl acetate,vinylpyrrolidones and vinyl group-containing siloxanes. Particularlypreferred (meth)acrylate polymers which can be used herein arecopolymers of at least one alkyl (meth)acrylate, in which the alkylmoiety has from 1 to 8 carbon atoms and at least one glycidyl(meth)acrylate containing up to 80 mol % particular from 5 to 50 mol %of units derived from said at least one glycidyl (meth)acrylate and,copolymers of at least one alkyl (meth)acrylate, in which the alkylmoiety has from 1 to 8 carbon atoms and at least one (meth)acrylic acidcontaining up to 50 mol %, in particular from 5 to 20 % mol on unitsderived from said at least one (meth)acrylic acid.

The alkyl (meth)acrylate polymer used herein is substantially free froma C--C double bond, and normally has a weight average molecular weightof from about 40,000 to about 1,500,000, preferably from about 100,000to 1,000.000.

The epoxy resin which can be used herein is an organic material havingan average of at least 1.8 vicinal epoxy groups per molecule, andnormally has a weight average molecular weight of from 100 to 10000.Examples of the epoxy resin include, for example, glycidyl ethers of aphenol such as Bisphenol A, Bisphenol F, resorcinol, phenol novolac andresorcinol novolac; glycidyl ethers of a polyhydric alcohol such asbutanediol, polyethylene glycol and polypropylene glycol; glycidylesters of a polycarboxylic acid such as terephthalic acid, isophthalicacid and tetrahydrophthalic acid; N,N-diglycidyl and N-alkyl-N-glycidylcompounds of an aromatic amine such as aniline isocyanurate; andalicyclic epoxides derived from an alicyclic olefin by oxidation of itsolefinic double bond or bonds, such as vinylcyclohexene diepoxide,3,4-epoxycyclohexylmethyl-3,4-dicyclohexane carboxylate and2-(3,4-epoxy)cyclohexyl-5,5-spiro (3,4-epoxy) cyclohexane-m-dioxane. Theepoxide compounds illustrated above may be used alone or in combination.Of these, diglycidyl ethers of a bisphenol are particularly preferred.As such preferred epoxy resin those which are commercially available are"Epikote" 828 having a molecular weight of 380, "Epikote" 834 having amolecular weight of 470, "Epikote" 1001 having a molecular weight of900, "Epikote" 1002 having a molecular weight of 1060, "Epikote" 1055having a molecular weight of 1350 and "Epikote" 100 having a molecularweight of 2900.

The epoxy resin is used in the adhesive composition for forming theadhesive layer 3 in an amount of normally from 5 to 2000 parts byweight, preferably from 100 to 1000 parts by weight, per 100 parts byweight of the (meth)acrylate polymer.

The photopolymerizable low molecular weight compound which can be usedherein is a compound having at least one carbon-carbon double bond whichis cross-linkable by irradiation with an energy beam such as anultraviolet ray and electron beam, and has a weight average molecularweight of, normally from 100 to 30,000, preferably from 300 to 10,000.Examples of preferred photopolymerizable oligomers are those havingfunctional groups such as hydroxy and carboxy, and include, for example,urethane acrylate, epoxy acrylate, polyester acrylate, polyetheracrylate, oligomer of (meth)acrylic acid and oligomer of itaconic acid.Of these, epoxy acrylate and urethane acrylate are particularlypreferred.

The photopolymerizable low molecular weight compound is used in theadhesive composition for forming the adhesive layer 3 in an amount ofnormally from 10 to 1000 parts by weight, preferably from 50 to 600parts by weight, per 100 parts by weight of the (meth)acrylate polymer.

The heat activatable latent curing agent for the epoxy resin is acompound having or potentially having at least two active hydrogen atomswhich are inactive at ambient temperature but are activated, whenheated, to react with the epoxy resin thereby effecting or promotingcuring of the epoxy resin. As such a heat activatable latent curingagent for the epoxy resin, use can be made of various onium salts, inparticular aliphatic and aromatic sulfonium salts, and other highmelting active hydrogen-containing compounds, alone or in combinationAmong others, aliphatic sulfonium salt are preferred.

The amount of the heat activatable latent curing agent used in theadhesive composition for forming the adhesive layer 3 may practically bewithin the range from 0.1 to 50 parts by weight, preferably from 1 to 10parts by weight, per 100 parts by weight of the epoxy resin,irrespective of the presence of any epoxy groups in the (meth)acrylatepolymer and/or photopolymerizable low molecular weight compound.

In addition to the heat activatable latent curing agent for epoxidefunctionality, a heat curing agent such as a polyisocyanate compound maybe incorporated in adhesive composition for the purpose of modifyingadhesive properties of the adhesive layer. When used, the amount of theheat curing agent may be normally from 0.1 to 30 parts by weight, andpreferably from 5 to 20 parts by weight, based on 100 parts by weight ofthe (meth)acrylate polymer.

The adhesive composition for forming the adhesive layer 3 furthercomprises a photopolymerization initiator for the photopolymerizable lowmolecular weight compound. Examples of the photopolymerization initiatorinclude, for example, benzophenone, acetophenone, benzoin, benzoin alkylether, benzil and benzil dimethylketal, alone or in combination. Ofthese, alpha-substituted acetophenones are preferred.

The photopolymerization initiator is used in the adhesive compositionfor forming the adhesive layer in an amount of normally from 0.1 to 10parts by weight, preferably from 1 to 5 parts by weight, per 100 partsby weight of the photopolymerizable low molecular weight compound.

Besides the above-mentioned components, the adhesive layer 3 may befurther incorporated with a leuco dye, a light scattering inorganiccompound, an expanding agent and an antistatic agent.

Examples of the leuco dye include, for example,3-[N-(p-tolylamino)-7-anilinofluoran and 4,4',4"-trisdimethylaminotriphenyimethane. The amount of the leuco dye, whenused, is normally from 0.01 to 10 parts by weight per 100 parts byweight of the (meth)acrylate polymer.

As the light scattering agent, suitable is a fine particulate inorganiccompound such as silica and alumina having a particle size of from 1 to100 μm, preferably from 1 to 20 μm. The amount of the light scatteringinorganic compound, when used, is normally from 0.1 to 10 parts byweight per 100 parts by weight of the (meth)acrylate polymer.

As the expanding agent, use can be made of higher fatty acids andderivatives thereof, silicone compounds and polyol compounds. The amountof the expanding agent, when used, is normally from 0.1 to 10 parts byweight per 100 parts by weight of the (meth)acrylate polymer.

Suitable as the antistatic agent are carbon black and anionic andcationic surfactants. The amount of the antistatic agent, when used, isnormally from 0.05 to 10 parts by weight per 100 parts by weight of the(meth)acrylate polymer.

If desired, electrical conductivity may be imparted to the adhesivelayer 3 by incorporating therein an electrically conductive substancesuch as gold, silver, copper, nickel, aluminum, stainless steel andcarbon. Such an electrically conductive substance is preferably used inan amount of from 10 to 400 parts by weight based on 100 parts by weightof the (meth)acrylate polymer.

The adhesive tape according to the invention can be prepared by coatingthe base sheet 2 on one surface thereof with a suitable adhesivecomposition for forming the desired adhesive layer 3 by means of agravure coater or a bar coater. If desired, necessary amounts ofnecessary components for forming the adhesive layer 3 may be dissolvedor dispersed in an appropriate solvent,and the resulting composition mayapplied on the base sheet 2.

The thickness of the adhesive layer 3 is normally from 3 to 100 μm, andpreferably from 10 to 60 μm.

The method for using the adhesive tape 1 of FIG. 1 in wafer processingwill now be described. Reference numbers refer to the accompanyingdrawings.

Where a strippable release sheet is provided, it is first removed, andthe adhesive tape 1 is placed, turning the adhesive layer 3 upward (FIG.1).

On the face side of the adhesive layer 3 is applied a semiconductorwafer A to be processed (FIG. 2).

In this state the wafer A is diced together with the adhesive layer 3 ofthe adhesive tape 1 into chips A₁, A₂, A₃ and so on by a suitable dicingmeans such as a dicing saw (FIG. 3) . In this step, the wafer A iscompletely diced into chips, whereas the adhesive layer 3 issubstantially diced. In other words, at least 50%, preferably 100% ofthe whole thickness of the adhesive layer is cut. The deeper the cut,the better.

The base sheet 2 is then expanded in two perpendicularly intersectingdirections within the plane of the sheet to facilitate the subsequentpicking up operation (FIG. 4). While the illustrated method involves astep of expanding, this step is not always necessary. In a case whereinthis stem is carried out, the base sheet 2 should naturally haveextensibility in machine and transverse directions . However, basesheets having no extensibility may also be used where no expansiontreatment is carried out.

Before the chips are picked up, the adhesive layer 3 is irradiated withenergy beam B to polymerize or cure the photopolymerizable compoundcontained in the adhesive layer 3 from the side of the base sheet 2 onwhich the adhesive layer 3 is not formed (FIG. 4 ) . As the energy beamultraviolet ray having a center wave length of about 365 nm ispreferred, and upon irradiation with such ultraviolet ray intensity ofradiation and irradiation time are preferably set within the ranges offrom 20 to 500 mW/cm² and from 0.1 to 150 seconds, respectively.

While the dicing step is carried out prior to the irradiation step inthe illustrated method, the irradiation may be effected prior to thedicing, in particular where no expansion is carried out. Furthermore,while the irradiation in the illustrated method is carried out beforethe chips come to a pick-up station where the pick-up operation iscarried out, the irradiation may be effected at the pick-up station, inparticular where the expansion carried out.

When the adhesive used herein is cured by irradiation with an energybeam, its adhesion strength to the wafer is increased well above a levelof its adhesion strength to the base sheet. This is believed to be dueto the presence of the epoxy resin in the adhesive.

Next, in a pick-up station, the wafer chips A₁, A₂, A₃ and so on aresuccessively pushed up by a pushing rod (not shown), picked up from thebase sheet 2 by a suitable means, such as a vacuum collet 6, as shown inFIG. 5, and collected in a wafer box (not shown). Since the curedadhesive has a greater adhesion strength to the wafer than to the basesheet 1, the wafer chips can readily be released together with the curedadhesive attached thereto from the base sheet 1. FIG. 6 depicts thewafer chip A₁ picked up by the vacuum collet 6. As seen from FIG. 6, thepicked up wafer chip A₁ carries a piece 3₁ of the diced and irradiatedadhesive layer 3. Since the cured adhesive attached to the wafer chipsis no longer sticky, the wafer chips collected in the wafer box do notstick to each other.

In the illustrated method the energy beam irradiation is effected atonce, but it may be done partially in several times. For instance,especially, in a case wherein the expansion of the base sheet has beencarried out, only the portion of the base sheet 1 corresponding to eachof the wafer chips A₁, A₂, A₃ and so on may be irradiated from the backside of the base sheet 1 by means of an irradiating tube, pushed up bythe same tube and picked up by the vacuum collect so that theirradiation may be carried out at the pick-up station.

The wafer chips A₁, A₂, A₃ and so on are then placed on a lead frame 7by means of a die-bonder (not shown) so that the respective pieces 3₁,3₂, 3₃ and so on of the adhesive layer may come in contact with the leadframe, and heated at a temperature of normally from 100° to 300° C.,preferably from 150° to 250° C., for a period of normally from 1 to 120minutes, preferably from 5 to 60 minutes so as to cause the curedadhesive contained in the pieces of the diced adhesive layer to againdevelop tackiness (FIG. 7). By this heating the adhesion strength of theadhesive layer to the wafer chip is increased to at least 1000 g/25 mm,and at the same time the wafer chips are firmly adhered to the leadframe 7 at an adhesion strength of substantially the same level. Thus,the wafer chips can be securely mounted the on lead frame 7.

In accordance with still another aspect of the invention there isprovided an adhesive composition comprising a (meth)acrylate polymerhaving at least 50 mol % of units derived from at least one(meth)acrylate, from 5 to 2000 parts by weight, based on 100 parts byweight of the (meth)acrylate polymer, of an epoxy resin having a weightaverage molecular weight of from 100 to 10000, from 10 to 1000 parts byweight, based on 100 parts by weight of the (meth)acrylate polymer, of aphotopolymerizable low molecular weight compound having a weight averagemolecular weight of from 100 to 30,000, a heat activatable latent curingagent for said epoxy resin and a photopolymerization initiator for saidphotopolymerizable low molecular weight compound.

The adhesive composition according to the invention provides an adhesivelayer which is curable by irradiation with an energy beam and the socured adhesive layer is capable of developing tackiness again byheating.

The invention will now be further described by the following examples,wherein parts are by weight unless otherwise specified.

EXAMPLE 1

    ______________________________________                                        (Meth)acrylate polymer having a solid                                                                    100 parts                                          content of 35% by weight and                                                                             (solid basis)                                      containing a copolymer of methyl acrylate                                     and glycidyl methacrylate, the copolymer                                      containing 20 mol % methyl acrylate                                           and having a molecular weight of about                                        300,000                                                                       Bisphenol diglycidyl ether based                                                                         600 parts                                          epoxy resin having a number average                                           molecular weight of 500 (supplied by                                          YUKA SHELL EPOXY K. K. under a trade                                          name of Epikote 834)                                                          Photopolymerizable epoxy acrylate                                                                        100 parts                                          oligomer having two C--C double bonds                                         and a molecular weight of 730 as                                              measured by GPC using polystyrene                                             as standard (supplied by SHIN-                                                NAKAMURA CHEMICAL Co, Ltd. under a                                            trade name of NK-ester EA-800)                                                Heat activatable latent curing                                                                            18 parts                                          agent for epoxy resins: aliphatic                                             sulfonium salt (supplied by ASAHI DENKA                                       KOGYO K. K. under a trade name of CP-66)                                      Photopolymerization initiator:                                                                            5 parts                                           2.2-dimethoxy-2-phenylacetophenone                                            ______________________________________                                    

An adhesive composition comprising the above-indicated components inindicated amounts was applied on a polyethylene film having a surfacetension of 32 dyne/cm and a thickness of 100 μm, and dried under heatingat 100° C. for 1 minute to prepare an adhesive tape having an adhesivelayer of a thickness of 30 μm.

A silicon wafer was applied onto the adhesive layer surface of theadhesive tape so prepared. The adhesive tape adhered to the siliconwafer at an adhesion strength of 360 g/25 mm, as measured in accordancewith JIS Z 0237. The adhesive layer was then irradiated with 200 mW/cm²of an ultraviolet ray for 2 seconds using a 80 W/cm high pressuremercury lamp. By this irradiation the adhesion strength between theadhesive layer and the silicon wafer increased to 900 g/25 mm, while theadhesion strength between the adhesive layer and the polyethylene basesheet after the irradiation was 60 g/25 mm.

The adhesive tape was employed in processing a silicon wafer as follows.A silicon wafer of the same quality having a diameter of 5 inches wasapplied onto the adhesive layer of the tape, and fully cut together withthe adhesive layer by means of a dicing saw supplied by DISCO Co.,Lid.at a cutting rate of 50 mm/sec into 5 mm square IC chips. Theadhesive layer was irradiated through the polyethylene base sheet withthe above-mentioned ultraviolet ray for 2 seconds. The IC chips werepicked up and then directly placed on a lead frame by means of adie-bonder so that pieces of the cured adhesive layer attached to thechips may come in contact with the lead frame. The chips were thenheated at a temperature of 170° C. for a period of 30 minutes wherebythey could be securely mounted on the lead frame.

EXAMPLE 2

    ______________________________________                                        (Meth)acrylate polymer having a solid                                                                    100 parts                                          content of 35% by weight and containing                                                                  (solid basis)                                      a copolymer of butyl acrylate and                                             acrylic acid, the copolymer containing                                        80 mol % of butyl acrylate and having                                         a molecular weight of about 500,000                                           Bisphenol diglycidyl ether based                                                                         400 parts                                          epoxy resin having a solid content of                                                                    (solid basis)                                      30% by weight and a number average                                            molecular weight of 900 (supplied by                                          YUKA SHELL EPOXY K. K. under a trade                                          name of Epikote 1001)                                                         Photopolymerizable urethane acrylate                                                                      70 parts                                          oligomer having two C--C double bonds                                         and a molecular weight of 5,000 as                                            measured by GPC using polystyrene                                             as standard (supplied by DAINICHISEIKA                                        COLOR & MFG. CO., LTD. under a trade                                          name of 14-33)                                                                Heat activatable latent curing                                                                            8 parts                                           agent for epoxy resins: aliphatic                                             sulfonium salt (supplied by ASAHI DENKA                                       KOGYO K. K. under a trade name of CP-66)                                      Photopolymerization initiator:                                                                            5 parts                                           2.2-dimethoxy-2-phenylacetophenone                                            Heat curing agent: aromatic                                                                               5 parts                                           polyisocyanate (supplied by TOYO INK                                          MFG. CO. LTD. under a trade name of                                           BHS-8515)                                                                     Electrically conductive filler:                                                                          750 parts                                          particulate nickel having a particle                                          size of 5 μm                                                               ______________________________________                                    

An adhesive composition comprising the above-indicated components inindicated amounts was applied on a polypropylene film having a surfacetension of 35 dyne/cm and a thickness of 60 μm, and dried under heatingat 100° C. for 1 minute to prepare an adhesive tape having an adhesivelayer of a thickness of 30 μm.

A silicon wafer was applied onto the adhesive layer surface of theadhesive tape so prepared. The adhesive tape adhered to the siliconwafer at an adhesion strength of 150 g/25 μmm. The adhesive layer wasthen irradiated with an ultraviolet ray as in Example 1. By thisirradiation the adhesion strength between the-adhesive layer and thesilicon wafer increased to 500 g/25 mm, while the adhesion strengthbetween the adhesive layer and the polypropylene base sheet after theirradiation was to 40 g/25 mm.

The adhesive tape was employed in processing a silicon wafer in themanner as described in Example 1. Good results were obtained as inExample 1.

Effect of the Invention

The adhesive layer of the adhesive tape according to the invention canbe cured by irradiation with an energy beam, and the so cured adhesivelayer is capable of developing tackiness again by heating. Furthermore,the adhesive tape has such a unique property that when it is adhered toa semiconductor wafer and irradiated with an energy beam, the adhesionstrength of the adhesive layer to the wafer becomes much higher thanthat to the base sheet. These properties of the adhesive tape accordingto the invention can be advantageously utilized in processing asemiconductor wafer according to the invention.

In the method according to the invention, the adhesive tape according tothe invention serves as a dicing tape for holding the wafer in positionduring the dicing step. Each piece of the diced and cured adhesivelayer, that is attached to each chip and capable of being tackified byheating, provides an adhesive required for securely mounting the chip onthe lead frame in the die-bonding step. Thus, the invention has solvedthe problems associated with the prior art including removal of anyresidual adhesive from diced IC chips and adequate provision of aseparate adhesive to the IC chips.

Modification of the Invention

According to one modification of the invention there is provided anadhesive tape comprising an adhesive layer composed of a (meth)acrylatepolymer, an epoxy resin, a photopolymerizable low molecular weightcompound, a heat activatable latent curing agent for said epoxy resinand a photopolymerization initiator for said photopolymerizable lowmolecular weight compound.

Such a base sheet free adhesive tape can be readily prepared by formingthe adhesive layer 3 on a release base 8 (FIG. 8), and can also be usedin processing a semiconductor wafer. Normally, a base sheet freeadhesive tape is supplied to customers in a form sandwiched between apair of release sheets. Upon use thereof in wafer processing, one of therelease sheets is removed to expose the adhesive layer 3; asemiconductor wafer to be processed is adhered to the adhesive layer;the adhesive layer is irradiated with an energy beam transmitted throughthe remaining release sheet; the wafer is diced into chips together withthe adhesive layer; the chips are picked up together with pieces of theirradiated and diced adhesive layer from the release sheet and placed ona lead frame so that the respective pieces of the adhesive layer adheredthereto may come in contact with said lead frame; and the pieces of thediced adhesive layer are caused to again develop tackiness by heating,thereby securely mounting the chips on said lead frame.

Thus, according to another modification of the invention there isprovided a method for using an adhesive tape comprising an adhesivelayer composed of a (meth)acrylate polymer, an epoxy resin, aphotopolymerizable low molecular weight compound, a heat activatablelatent curing agent for said epoxy resin and a photopolymerizationinitiator for said photopolymerizable low molecular weight compound,said method comprising the steps of adhering a semiconductor wafer tosaid adhesive tape formed on a release sheet, irradiating the adhesivelayer with an energy beam dicing said wafer into chips together with theadhesive layer, picking up the chips together with pieces of the dicedadhesive layer adhered thereto from said release sheet, placing thechips together with pieces of the diced adhesive layer adhered theretoon a lead frame so that the respective pieces of the adhesive layer maycome in contact with said lead frame, and causing the pieces of thediced adhesive layer to again develop tackiness by heating, therebysecurely mounting the chips on said lead frame.

What is claimed is:
 1. An adhesive composition for forming an adhesivelayer which is curable by irradiation with an energy beam and whereinthe so cured adhesive layer is capable of developing tackiness again byheating, said composition consisting essentially of:(a) 100 parts byweight of a (meth)acrylate polymer substantially free from C--C doublebonds and having at least 50 mol % of units derived from at least one(meth)acrylate and a weight average molecular weight of from about40,000 to 1,500,000; (b) from 400 to 2,000 parts by weight of an epoxyresin having an average of at least 1.8 vicinal epoxy groups permolecule and a weight average molecular weight of from 100 to 10,000;(c) from 10 to 1,000 parts by weight of a photopolymerizable lowmolecular weight compound having at least one C--C double bond andhaving a weight average molecular weight of from 100 to 30,000; (d) aheat activatable latent curing agent for said epoxy resin; and (e) aphotopolymerization initiator for said photopolymerizable low molecularweight compound.